Optical fiber is useful in communication networks to transmit digital and analog information via modulated optical signals. In a typical optical network, an optical fiber receives source light from a semiconductor laser, such as a vertical cavity surface emitting laser (“VCSEL”), a Fabry-Perot laser, or a distributed feedback laser (“DFB”). The optical fiber guides the light to an optical detector, which converts optical signals into corresponding electrical signals. Communications equipment processes the electrical signals and decodes the information.
One type of communications equipment that converts optical signals into electrical signals is an optical detector. Optical detectors typically respond to light over a dynamic range. That is, over a range of intensities, most optical detectors produce an electrical signal that linearly corresponds to the intensity of the light that is incident upon the optical detector. If the intensity of the optical signal that is incident upon the optical detector is higher than its dynamic range, the optical detector's performance can suffer. Consequently, assorted conventional devices are available to manage power in an optical network and keep the intensity of the optical signal within an optical detector's dynamic range.
In order to manage power, an optical network can include an optical attenuator positioned in the optical path between an optical source and an optical detector. So deployed, an optical attenuator can reduce the intensity of an optical signal and place it within the dynamic range of an optical detector. That is, conventional optical attenuators generally are power management devices that adjust the strength of an optical signal to optimize an optical detector's response to the optical signal. The conventional art includes numerous types of optical attenuators specific to this purpose. One type of conventional attenuator includes a small-diameter spool around which optical fiber is wrapped. The degree of attenuation is a function of the number of turns of optical fiber on the spool.
While attenuators find conventional utility for managing optical power, other conventional devices are generally used to address optical reflections in an optical network. In point-to-multipoint optical networks, a single light source is optically coupled to multiple fiber optic branches. At any time, some of the branches may be in service, actively transmitting optical signals to a destination, such as an optical detector at a subscriber premises. At the same time, other branches may be spares, held in reserve for network expansion. That is, the typical optical network includes active optical fibers transmitting information from a source to an optical detector and other reserve optical fibers that propagate or carry optical signals to a dead end. In other words, the reserve optical fibers are openly terminated, or are commonly said to be “unterminated.” The reserve optical fibers are often optically coupled on one end to a source such as another active optical fiber and remain open on another end. In this network configuration, light can propagate in an optical fiber with an open end face and, when incident on the end face, internally reflect off the open end face. This reflected light can then back propagate in the optical fiber and can interfere with network performance. Such stray light reflections from openly terminated optical fibers in an optical network can cause performance problems.
Specifically, stray reflections from openly terminated optical fibers can impair the performance of an optical communications link by interfering with an optical detector, for example. When a stray reflection propagates in an optical fiber at the same time with another optical signal, an optical detector can confuse the two optical signals. That is, when an optical network concurrently transmits a reflected optical signal and an optical signal supporting desired communication information to an optical detector, the signal-to-noise ratio of the network can suffer.
Stray reflections can also impair the performance of a semiconductor laser. When light reflects off an end face of an optical fiber and back into a semiconductor laser, the back reflected light can interference with the laser's operation. For example, the back reflected light can destabilize the laser's lasing cavity.
An open end face of an optical fiber can internally reflect approximately four percent of the forward propagating light that is incident upon it. A number of conventional approaches have been taken to address such fiber optic back reflections. Isolators are optical devices that suppress back reflections by allowing light to flow in one direction but not in the other. Isolators are often coupled to high performance lasers and generally are considered too expensive for routine fiber optic applications.
Another conventional approach includes adapting the end face of an optical fiber to either minimize the intensity of a back reflection or to prevent back reflected light from back propagating in an optical fiber. Coating the end face with an antireflective film or patterning it with microstructures can minimize the intensity of back reflected light. Cleaving an optical fiber at an angle can produce an end face that deflects light away from the core of an optical fiber so that the optical fiber does not significantly back propagate internal reflections. Implementing these approaches can be cumbersome or expensive, particularly under field conditions.
Another conventional approach includes permanently deforming an optical fiber, for example heating it to diffuse its core into its cladding or by forming a permanent kink in it. These processes typically require special equipment and are irreversible.
Yet another approach includes tying an optical fiber into a knot near an end face of the optical fiber. The knot attenuates the light that is propagating in the optical fiber towards the end face. Although the knot approach is generally convenient and can be implemented without special equipment, it has significant shortcomings. Since the distortion of the optical fiber in the knot is generally uncontrolled, the knot may impose significant and uncontrolled mechanical stress on the optical fiber. Such stress can shorten the life of the optical fiber or cause it to fracture. If a technician unties the knot and couples the optical fiber to an optical detector, the formerly-knotted section of optical fiber can prematurely fail. For example, the optical fiber can break without warning several years after the technician coupled it to an optical detector. Stresses associated with the knot can also cause an optical fiber to catastrophically fracture while it is knotted, for example before it is coupled to an optical detector. If the knotted optical fiber catastrophically fractures in the field resulting in a shatter rather than a clean break, the fracture can induce back reflections. Stresses can also induce micro fractures that cause back reflections, even without catastrophic failure of the optical fiber. If a technician does not tie the knot tight enough, sufficient light may propagate through the knotted section of optical fiber to impair network performance. Furthermore, the possibility exists for vibrations and cyclic heating and cooling to loosen the knot or cause it to come untied.
Another problem with the knotted-fiber approach is that light generally exits the core of the optical fiber over a very short length of optical fiber. For applications involving high-power lasers, for example pump lasers and cutting lasers, the power density of the exiting light in this short section can be high. Over time, the potential exists for such power density to damage the optical fiber's coating or sheathing.
What is needed is a capability for managing back reflected light in an optical system so an end face of an optical fiber does not produce internal reflections that impair the performance of the system. This capability should be predictably reliable and should be conveniently implemented with minimal equipment. Such a capability would facilitate optical networks in cost-sensitive applications, such as fiber-to-the-home.